We studied the effects of exogenous spermidine (Spd)on plant growth and nitrogen metabolism in two cultivars of tomato (Solanum lycopersicum)that have differential sensitivity to mixed salinity-alkalinity stress: 'Jinpeng Chaoguan' (salt-tolerant)and 'Zhongza No. 9' (salt-sensitive). Seedling growth of both tomato cultivars was inhibited by salinity-alkalinity stress, but Spd treatment alleviated the growth reduction to some extent, especially in 'Zhongza No. 9'. Exogenous Spd may help reduce stress-induced increases in free amino acids, ammonium (NH4 +)contents, and NADH-dependent glutamate dehydrogenase (NADH-GDH)activities; depress stress-induced decreases in soluble protein and nitrate content; and depress nitrate reductase, nitrite reductase, glutamine synthetase (GS), NADH-dependent glutamate synthase (NADH-GOGAT), glutamate oxaloacetate transaminase (GOT), and glutamate pyruvate transaminase (GPT)activities, especially for 'Zhongza No. 9'. Based on our results, we suggest that exogenous Spd promotes the assimilation of excess toxic NH4 + by coordinating and strengthening the synergistic action of NADH-GDH, GS/NADH-GOGAT, and transamination pathways, all during saline-alkaline stress. Subsequently, NH4 + and its related enzymes (GDH, GS, GOGAT, GOT, and GPT), in vivo, are maintained in a proper and balanced state to enable mitigation of stress-resulted damages. These results suggest that exogenous Spd treatment can relieve nitrogen metabolic disturbances caused by salinity-alkalinity stress and eventually promote plant growth.